The present invention relates to a method of reproducing or reading, by means of a laser beam, signals which have been recorded in an optical disk.
A method has been proposed in the specification of U.S. Pat. No. 4,569,038 for recording data at a high density in an optical disk, by making use of V-shaped track grooves. More specifically, in this proposed method, a V-shaped track groove having slant side surfaces is formed in an optical disk at a pitch which is substantially the same as the pitches of recording tracks in conventional optical disks. In this method, signals are recorded in both slant side surfaces so that the recording density is substantially doubled. In the reproduction of the recorded signals, the beam portion reflected by the optical disk is received by a photodetector. However, the reproducing device does not reproduce whole the reflected beam but only the peripheral portion of the reflected beam. By suitably selecting the reproduction region in the beam, it is possible to reduce any crosstalk from the adjacent slant side surface of the V-shaped track groove. This is one of advantageous features offered by the V-shaped track groove. The reflected beam involves not only a reproduction with small crosstalk component but also a region which has a large crosstalk component though the light quantity is small. Therefore, the reproduction of signal is preferably conducted by means of a photodetector which is capable of reproducing signals only in the region having small crosstalk component. In order to obtain a sufficiently low level of crosstalk, e.g., -36 dB, it is necessary to ensure a high degree of accuracy not only in design but also in installation of the photodetector.